The present invention relates to a method and an apparatus for refining metal carbide containing free carbon, particularly to a method of burning and removing free carbon contained in a fine metal carbide powder such as powders of carbide of silicon, titanium, tantalum, niobium, tungsten, vanadium and the like and an apparatus used therefor.
Heretofore, metal carbide has been produced mainly by allowing carbon to react with a metal or a metal oxide. In a metal carbide produced by such a method, there are unavoidably mixed free carbon combined with the metal carbide. Therefore, a step of refining a metal carbide by removing free carbon has become an important process in producing a metal carbide.
For removing free carbon contained in a metal carbide, there have hitherto been known methods such as (a) gravity concentration method using the difference between specific gravities of a metal carbide and free carbon, (b) floatation method using the difference between affinities to air of a metal carbide and free carbon, and (c) burning method of removing free carbon using the difference between oxidation speeds of a metal carbide and free carbon. Among these methods, the gravity concentration method (a) has a disadvantage of requiring a prolonged time for separation unless the metal carbide has relatively coarse particles and the gravity difference is large. While, the floatation method (b) has been disclosed, for example, in Japanese Patent Application Publication No. 24,480/76, it has a disadvantage that the efficiency of separation decreases unless particle sizes of metal carbide to be treated are set in a relatively narrow range. Moreover, either the method (a) or (b) has a disadvantage that metal carbide and free carbon have to be preliminarily separated to their unit particles before being subjected to respective treatment. However, the burning method (c) of removing free carbon is capable of removing free carbon uniformly even when metal carbide to be treated is fine particles which are difficult to be preliminarily separated to unit particles of metal carbide and free carbon.
For the burning method of removing free carbon contained in a metal carbide, there can be conceived such methods as (1) a method of burning the free carbon contained in the metal carbide on a dish type container in a furnace, (2) a method of burning the free carbon contained in the metal carbide in a multi-stage furnace or a revolving furnace such as a rotary kiln and we conceived of (3) a method of burning the free carbon contained in the metal carbide under fluidization of the metal carbide. Among these methods, either the method (1) or (2) has drawbacks that efficiency of contacting free carbon with an oxidizing gas is low so that a prolonged time is necessary for burning and removing free carbon sufficiently, and that the loss of metal carbide due to oxidization thereof is large. Therefore, both the methods (1) and (2) have not been suitable methods for refining a great quantity of metal carbide. Meanwhile, we have found out that the method (3) of fluidizing metal carbide has extremely high efficiency of contacting metal carbide with an oxidizing gas, however, it has drawbacks in that fine metal carbide powders are liable to scatter unless the grain size of the metal carbide powders is set in a coarse range and that a uniform fluidizing state is scarcely obtained because of formation of agglomerated masses of the metal carbide powders due to the adhering property of formed oxides at surfaces of metal carbides during the burning process. Hence, it is not suited particularly to burning and removing of free carbon contained in fine metal carbide powders.